As shown in FIG. 1, distributed voice conferencing, also known as distributed teleconferencing, allows a number of people in the same acoustical space, such as a number of people within the same conference room, to participate in a teleconference with each person utilizing the microphone and loudspeaker of their own mobile terminal 10 to transmit and receive audio signals during the teleconference. In order to establish a distributed voice conference, a wireless network may be established between the mobile terminals that are resident within the same acoustical space.
As shown at Location A of FIG. 2, one of the mobile terminals may serve as a host device 12 and, as such, communicates with the other mobile terminals 10 within the same acoustical space. Based upon audio signals received by the microphones of the mobile terminals and provided to the host device, an enhanced uplink signal may be generated by the host device utilizing a mixing algorithm. The enhanced uplink signal may then be provided to a central conference server 14, such as a conference bridge, and/or to another remotely located mobile terminal. In the downlink direction, the host device may receive an audio signal from the network, such as from the central conference server, and may then share the audio signal with the other mobile terminals within the acoustic space. The audio signal may then be reproduced by the respective loudspeakers of the mobile terminals within the acoustic space.
As shown at location C of FIG. 2, the central conference server, such as a conference bridge, may facilitate distributed conferencing functionality by serving as a host device for the mobile terminals at Location C. In this regard, the mobile terminals at location C may contact the central conference server and the mobile terminals at location C maybe determined to be within the same acoustical space. In this regard, the central conference server may automatically detect that the mobile terminals are within the same acoustical space. Alternatively, the mobile terminals may provide an indication, such as via dual-tone multi-frequency (DTMF) signaling, indicative of the mobile terminals being within the same acoustical space.
Distributed conferencing may provide improved speech quality on the far end since the microphones of the mobile terminals are located near the participants. Additionally, at the near end, less listening effort may be required since multiple loudspeakers may be utilized to reproduce the audio. In this regard, the use of several loudspeakers may reduce distortion levels since the loudspeaker output may be maintained at a relatively low level compared to the level that must be maintained in an instance in which a single loudspeaker is utilized for a conference room. Distributed conference audio also allows a determination to be made as to who is currently speaking in the conference room.
The audio signals generated by the loudspeaker may be picked up or received by the microphone of a mobile terminal and perceived as an undesirable echo at the far end. This phenomenon is referred to as acoustic echo as a result of the acoustic coupling between the transducers. Acoustic echo control (AEC) has been developed in order to attenuate the acoustic coupling between the audio signals reproduced by the loudspeaker and the audio signals received by the microphone of the same device. AEC generally utilizes an adaptive filter positioned between the downlink and uplink signals followed by a residual echo suppression unit. In order to attenuate the echo, the AEC must have knowledge of or access to the downlink signal that causes the echo when reproduced by the loudspeaker. In distributed voice conferencing, each mobile terminal may include AEC in order to attenuate the echo created by the acoustic coupling between the audio signals generated by the loudspeaker and the audio signals received by the microphone of the same device so as to provide an acceptable communication experience for the participants.
Distributed voice conference systems have been developed that utilize spatial audio processing and multi-channel audio reproduction; instead of monophonic downlink reproduction. Spatial sound may improve speech intelligibility and speaker identification by providing spatial cues for the listener in order to segregate the remote speakers more easily. The spatial cues may be created utilizing time and amplitude panning with digital filtering prior to reproduction.
In regards to spatial audio processing, each mobile terminal of a distributed group drives its loudspeaker to reproduce a different downlink signal. One example is illustrated in FIG. 3. In this example, the speech signal originating with mobile terminal A is reproduced only by the loudspeaker of mobile terminal B. As such, while the user of mobile terminal A is speaking, the audio signals may be reproduced by the loudspeaker of mobile terminal B and the acoustic echo control of mobile terminal B may cancel the acoustic echo received by its own microphone. However, the microphone of mobile terminal C also captures the audio signal reproduced by the loudspeaker of mobile terminal B. As mobile terminal C did not reproduce the audio signal, mobile terminal C does not attenuate the echo and, instead, causes the echo to be mixed with the uplink signal that is transmitted to mobile terminal A such that the user of mobile terminal A hears an echo. Similarly, audio signals provided by mobile terminal B may be reproduced by the loudspeaker of mobile terminal C. The echo received by the microphone of mobile terminal C may be cancelled, such as by acoustic echo cancellation provided by mobile terminal C, but the same audio signals may also be received by the microphone of mobile terminal B. Since the audio signals did not originate from the loudspeaker of mobile terminal B, mobile terminal B may treat the received audio signals as the product of a local sound source and mix these audio signals with the uplink signal that is transmitted to mobile terminals A and D so that the user of mobile terminal D experiences an echo.